Re: rseq: How to test for compat task at signal delivery

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Jun 26, 2018, at 3:32 PM, Andy Lutomirski luto@amacapital.net wrote:

> On Tue, Jun 26, 2018 at 11:45 AM Mathieu Desnoyers
> <mathieu.desnoyers@efficios.com> wrote:
>>
>> ----- On Jun 26, 2018, at 1:38 PM, Mathieu Desnoyers
>> mathieu.desnoyers@efficios.com wrote:
>>
>> > Hi Andy,
>> >
>> > I would like to make the behavior rseq on compat tasks more robust
>> > by ensuring that kernel/rseq.c:rseq_get_rseq_cs() clears the high
>> > bits of rseq_cs->abort_ip, rseq_cs->start_ip and
>> > rseq_cs->post_commit_offset when a 32-bit binary is run on a 64-bit
>> > kernel.
>> >
>> > The intent here is that if user-space has garbage rather than zeroes
>> > in its struct rseq_cs fields padding, the behavior will be the same
>> > whether the binary is run on 32-bit or 64 kernels.
>> >
>> > I know that internally, the kernel is making a transition from
>> > is_compat_task() to in_compat_syscall().
>> >
>> > I'm fine with using in_compat_syscall() when rseq_get_rseq_cs() is
>> > invoked from a system call, but is it OK to call it when it is
>> > invoked from signal delivery ? AFAIU, signals can be delivered
>> > upon return from interrupt as well.
>> >
>> > If not, what strategy do you recommend for arch-agnostic code ?
>>
>> I think what we're missing here is a new "is_compat_frame(struct ksignal *ksig)"
>> which I could use in the rseq code. I'll prepare a patch and we can discuss
>> from there.
>>
> 
> That sounds about right.
> 
> I'm confused, though.  Wouldn't it be more consistent to just segfault
> if the high 32 bits are not clear when rseq transitions to a 32-bit
> context?  If there's garbage in 64-bit mode, the program will crash.
> Why should 32-bit mode be any different?

Currently, if a 32-bit binary puts garbage in the high bits of
start_ip, post_commit_offset, and abort_ip in

include/uapi/linux/rseq.h:

struct rseq_cs {
        /* Version of this structure. */
        __u32 version;
        /* enum rseq_cs_flags */
        __u32 flags;
        LINUX_FIELD_u32_u64(start_ip);
        /* Offset from start_ip. */
        LINUX_FIELD_u32_u64(post_commit_offset);
        LINUX_FIELD_u32_u64(abort_ip);
} __attribute__((aligned(4 * sizeof(__u64))));

A 32-bit kernel just never reads the padding, thus in reality acting
as if those were zeroes. However, a 64-bit kernel dealing with this
32-bit compat task will read that padding, handling those as very
large values.

We need to improve that by introducing a consistent behavior across
native 32-bit kernels and 32-bit compat mode on 64-bit kernels.

There are two ways to achieve this: either the 32-bit kernel validates
the padding by killing the process if padding is non-zero, or the
64-bit kernel treats compat mode by zeroing the high bits of padding.

If we look at system call interfaces in general, I think the usual
approach is to clear the top bits whenever a value read from a
compat task ends up being used as a pointer. This is why I am tempted
to go for the "clear high bits" approach rather than killing the task.

Also, validating that the top 32-bit is zeroes from a native 32-bit
kernel requires extra loads, whereas not caring about their content
is free, which makes me slightly prefer an approach where 32-bit
compat mode on 64-bit kernel just clears the top bits.

Thoughts ?

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com